An Emerging View on Early Land Use

More than 20 years ago, analyses of greenhouse gas concentrations in ice cores showed that downward trends in CO2 and CH4 that had begun near 10,000 years ago subsequently reversed direction and rose steadily during the last several thousand years. Competing explanations for these increases have invoked either natural changes or anthropogenic emissions. Reasonably convincing evidence for and against both causes has been put forward, and the debate has continued for almost a decade. Figure 1 summarizes these different views.

An August 2011 special issue of the journal The Holocene will help to move this discussion forward. All scientists who have been part of this debate during the last decade were invited to contribute to the volume. The list of those invited was well balanced between the two views, both of which are well represented in the issue. The papers have recently begun to come online, but unfortunately behind a paywall.

Arguably, the most significant new insight emerging from this issue comes from several papers that converge on a view of pre-industrial land use that is very different from the one that has prevailed until recently. Most previous modeling simulations relied on the simplifying assumption that per-capita clearance and cultivation remained small and nearly constant during the late Holocene, but historical and archeological data now reveal much larger earlier per-capita land use than used in these models. The emergence of this view was reported in several presentations at a March 2011 Chapman Conference, and it has attracted recent attention both in Nature and Science News. The following article summarizes this new evidence.

Historical data on land use extending back some 2000 years exists for two regions — Europe and China. In a 2009 paper, Jed Kaplan and colleagues reported evidence showing nearly complete deforestation in Europe at mid-range population densities, but very little additional clearance at higher densities. Embedded in this historical relationship was a trend from much greater per-capita clearance 2000 years ago to much smaller values in recent centuries. Similarly, a Holocene special-issue paper by Ruddiman and colleagues pointed to a pioneering study of early agriculture in China published in 1937 by J. L. Buck. Paired with reasonably well-constrained population estimates that extend back to the Han dynasty 2000 years ago, these data show a 4-fold decrease in per-capita land area cultivated in China from that time until the 1800’s.

These two re-evaluations of per capita land use have important implications for global pre-industrial carbon emissions. A special issue paper by Kaplan and colleagues used the historical relationships from Europe to estimate worldwide clearance, with smaller per-capita land needs in tropical regions due to the longer growing season that allows multiple crops per year. Their model simulated major forest clearance thousands of years ago not just in Europe and China, but also in India, the Fertile Crescent, Sahelian Africa, Mexico and Peru. The pattern of clearance is nicely shown in a time-lapse sequence available in the Science News article cited above. Kaplan and colleagues estimated cumulative carbon emissions of ~340 GtC (1 Gt = billion metric tons) before the industrial-era CO2 rise began in 1850. This estimate is 5 to 7 times larger than those based on the assumption that early farmers cleared forests and cultivated land in the small per-capita amounts typical of recent centuries.

Over millennial time scales, roughly 85% of CO2 emissions to the atmosphere end up in the deep ocean. As a result, Kaplan’s estimated 340 Gt of early anthropogenic carbon emissions to the atmosphere would have resulted in a total pre-industrial CO2 increase of ~24 ppm (340 Gt divided by 14.2 Gt per ppm). A mismatch in timing remains, however, between the early rise of the ice-core CO2 trend and the later rise of Kaplan’s carbon-emissions estimate. One possibility currently being investigated by Kaplan and colleagues is larger early per-capita burning by farmers (and those cultures still in the hunter-gatherer stage).

A similar story of decreasing per-capita land use holds for farming practices that generate methane. The paper by Ruddiman and colleagues cites a 1997 study by Ellis and Wang in Agriculture, Ecosystems, and Environment (61: 177-193) reporting a 4-fold decrease from 1000 to 1800 AD in the per-capita size of rice paddy fields in the lower Yangtze River valley. Because of ongoing population growth and the lack of additional arable land, farmers were forced to produce rice on ever-smaller land holdings, resulting in the typical ‘garden-style’ Chinese agriculture.

For longer time scales, an in-press paper by Fuller and colleagues on ‘The contribution of rice agriculture and livestock pastoralism to prehistoric methane levels: an archeological assessment’ assembled archeological evidence from hundreds of well-dated sites showing the spread of irrigated rice across southern Asia between 5000 and 1000 years ago. Based on modern regional relationships, they assumed that rice farming in each region subsequently filled in with the log of population density. Combining the first arrival of rice and the subsequent infilling, Fuller and colleagues projected the progressive increase in the total area of southern Asia devoted to irrigated rice.

Their estimate showed a rising exponential trend in total area that reached more than 35% of the modern value by 1000 years ago, even though the population in the rice-growing areas of Asia at that time was only 5-6% of modern levels. This mismatch again indicates much greater per-capita land use early in the historical era than in later pre-industrial time. According to this analysis, the increase in CH4 emissions from rice irrigation can account for most of the CH4 rise measured in ice cores between 5000 and 1000 years ago. Fuller and colleagues also mapped the first arrival of domesticated livestock across Asia and Africa and found that a major expansion of pastoralism into wet areas with high carrying capacities began after 5000 years ago. They noted that this spread of livestock would also have made a major contribution to anthropogenic methane emissions and atmospheric concentrations but did not attempt to estimate the amount.

The evidence in all of these recent papers converges on the same conclusion: the simplifying assumption of constant per-capita land use used by most previous modeling studies has ignored both historical data and the wide range of contrary evidence assembled by scientists in archeology and related disciplines who do the slow ‘dirty-boots’ field work needed to unravel the real history of human effects on the land. This field-based view was synthesized long ago by Ester Boserup’s seminal work in the 1960’s through the 1980’s. She concluded that the major decrease in per-capita land use through the middle and late Holocene occurred because population growth and the encroachments of neighbors forced farmers to innovate new methods to produce food for their families from less and less land. These papers in the special issue make it clear that future attempts to model past land use should avoid the assumption of constant and small per-capita cultivation and clearance.

This emerging view bears on a current discussion over whether or not to designate an ‘Anthropocene’ interval (a time of major human influence on Earth’s system) and, if so, when to place its beginning. Although most opinion seems to favor using the industrial era (the last two centuries or less) as the start, this new evidence offers a different perspective. Forest clearance for cultivation and pastureland is the largest transformation of Earth’s surface that has occurred to date. If well over half of this key transformation occurred prior to the industrial era, then an argument can be made for placing the start of the anthropocene at an earlier time. One possible solution would be to designate two stages: an ‘early anthropocene’ (a time of slow but growing and cumulatively large transformations that began ~7000 years ago for CO2 and ~5000 years ago for CH4) and a ‘late anthropocene’ to mark the many accelerating changes of the industrial era.

Other special-issue papers also point to a revised interpretation of a related kind of evidence that bears on early deforestation— the meticulous analyses of the carbon-isotopic composition of CO2 in ice-core air bubbles by the Bern group. Elsig et al. concluded in 2009 in an article in Nature that the small (~0.05o/oo) amplitude of the δ13CO2 decrease during the last 7000 years constrains net emissions of terrestrial carbon to ~50 GtC (one Gt is a billion tons), if fully equilibrated with the deep ocean. As part of their proposed balancing of various carbon sources and sinks, they estimated an anthropogenic contribution of ~50 GtC to the δ13CO2 trend, equivalent to a CO2 increase of 3.5 ppm.

But the mass balance calculation in Elsig et al. entailed the questionable assumption that only 40 Gt of carbon has been buried in boreal peats during the last 7000 years, yet this value lies well below a long-respected estimate of 300 GtC by Eville Gorham (e.g. Ecological Applications 1: 182-195, 1991; Gajewski et al., Global biogeochemical Cycles 15: 297-310; 2001). A new analysis by Zicheng Yu in the special issue takes into account both the initial burial of peat carbon and, for the first time in any study, the subsequent decomposition and release of peat carbon after burial. Yu arrives at an estimate of ~300 Gt of carbon burial in peat during the last 7000 years.

This much higher value (~300 GtC versus 40 GtC) requires much larger offsetting emissions of terrestrial carbon to satisfy the overall δ13CO2 constraint, but the additional carbon is unlikely to have come from natural sources. Model studies have, on average, placed the net carbon balance caused by natural changes in monsoon vegetation and carbon fertilization close to the 30 Gt size estimated by Elsig and colleagues. These changes cannot account for the emissions needed to offset the much larger amount of carbon buried in peat.

The only remaining source left is anthropogenic emissions. The resulting estimate of >300 GtC of preindustrial anthropogenic emissions is in the same ballpark as the land-use simulation estimate from Kaplan and colleagues. If the earlier (Gorham) and more recent (Yu) estimates of large carbon burial in boreal peat are correct, the small negative δ13CO2 trend during the last 7000 years is not an argument against the early anthropogenic hypothesis, but rather an argument in its favor.

The two estimates of a pre-industrial anthropogenic CO2 increase of as much as 24 ppm are much larger than previous estimates of 3-5 ppm, but still short of the 40 ppm proposed in the early anthropogenic hypothesis. However, another factor that would have contributed to the pre-industrial anthropogenic total was CO2 feedback from an ocean kept warmer by agricultural emissions of CO2 and CH4 to the atmosphere. A special-issue paper by Kutzbach and colleagues estimates a contribution of as much as 9 ppm from the reduced solubility of CO2 in an ocean warmed by the early anthropogenic CO2 and CH4 emissions to the atmosphere. This, and other possible feedbacks from the ocean, put the total pre-industrial CO2 effect at >30 ppm, closer to the 40 ppm in the original hypothesis.

Several papers in the special issue continue to favor a natural explanation for the late-Holocene CO2 and CH4 trends, so the debate is not over. Still, the new evidence points the way toward three avenues of exploration that promise to deliver a resolution of this issue: (1) more thorough investigation of historical records of pre-industrial land use; (2) additional archeological work to fill in gaps in spatial/temporal coverage of the spread of agriculture, and (3) further modeling work to transform historical and archeological data into quantitative estimates of the effects of early agriculture on atmospheric CO2 and CH4 concentrations.

98 Responses to “An Emerging View on Early Land Use”

This discussion that provides an example of science in action that is courteous, informative, and wise. Thanks to the patience that provides responses and updates. I don’t know why my mind (only partially educated in science) is so stubborn, but I finally have a better idea about some of the background, such as the Milankovich cycle, thanks to you all.

I find this discussion broadening; it’s easy to visualize/imagine what is being talked about and see how the various influences develop and how partial information can be accumulated and tested. I do wonder if when natural cycles are disturbed traces of influence of what would happen absent all our interference persist; of course angles, orbits, and distances and are still there, so perhaps I don’t know what I’m trying to say. Once it is changed, how powerful is the influence that might return it to “normal”? (Setting aside that perhaps existential “normal” doesn’t really exist; what we have is a concatenation of effects.)

My primary involvement has been communication, and I am always hoping for ways to talk about this that reach outside the very small circle of people who know enough and those who can believe six impossible things before breakfast. The stubborn persistence of bad thinking is perfectly illustrated by a comment on Mooney’s latest (science of anti-science over at MotherJones):

ps. you may assume that I really carefully all relevant comments and look at links, despite my inability to grasp some of the science and keep all the detail in mind.

But real reason for this return is to mention:

1. In my childhood (50s) the neighboring area was burned over annually, and it was not long ago that regular prophylactic fires were common, including forests. I know this is not the period being discussed, but may illustrate how people think and manage.

2. If you have ever hiked through various forms of scrub, you can easily imagine that people would clear it for simple convenience, not to say survival. Without all mod cons various effects such as pest control would also apply.

Re comment #49: Paragraphs 12-16 in my original post dealt with this problem (not easy to explain!). The d13CO2 index complied by Elsig et al (Tom Stocker’s group) does indeed indicate a small net release of terrestrial carbon over the last 7000 years. However that index is the sum of many separate inputs and outputs, including natural C emissions from areas with weakening monsoons, C storage due to increased CO2 fertilization as CO2 rose by 22 ppm, and C removed from the atmosphere and stored in peat. My opinion is that the Elsig et al. estimate of storage in peat is far too low, and a more realistic (much larger) estimate would require a large offsetting input from other terrestrial emissions to yield that small net d13CO2 release. I argue that anthropogenic emissions are the most likely source. As an imperfect analogy, think of seeing an iceberg and trying to estimate its total volume from the part sticking up above the water line. Allowing for a more realistic estimate of larger peat burial is a bit like looking underwater to see how big the iceberg really is.

Re comment #50: The Nevle article is not online yet, but you could read Dull et al. DOI: 10.1080/00045608.2010.502432 for a sense of the debate. However, by well-respected estimates, only 7 million of 55-60 million early Americans lived in North America, so the main American contribution would be from Mexico and South America, where agriculture developed and spread earlier. And of course an even larger contribution from the several hundred million people in China, India, and Europe. Because experts disagree on fire, I don’t mean to push this early burning idea too hard, but simply to point it out as a possible explanation for the early CO2 rise (in addition to higher per-capita cultivation). Jed Kaplan is now working on a model of earlier burning based on modern patterns: see Thonicke et al (http://www.biogeosciences.net/7/1991/2010). People burn extensively at low population density, less when the density passes a threshold, and hardly at all when the density becomes very high. That would help boost early CO2 emissions.

RE: #47, Jim Cross -I believe if you look into this, instead of making assumptions, you’ll find that there is substantial evidence that such practices occurred – and may have been a significant factor in the disappearance of megafauna in both the Old world, and the new. Current human activities are a great example of how, despite scientific evidence that our activities may not be in our best interest, we do them anyway. Paleolithic people didn’t have scientist to tell them that their practices were unwise, either. We are talking about highly mobile cultures that likely failed to perceive the ultimate negative outcome of what they were doing.

If you have increasing farming + increasing CO2, you can also imaging cause and effect the other way around. Humanity profited from the better weather and or favourable CO2 levels for crops, and farmed more. Or you can have another common cause pushing both. Some sources put farming earlier – starting 8,500 BC. So there is plenty of scope to fit an effect to a developement in farming be it old Testiment or 20th Century.

nor that they were “a significant factor in the disappearance of megafauna” – I know this is a popular belief but it ain’t necessarily so.

To make it clear – I think it highly likely that land-clearing and other early agricultural practices may have had an impact on ghg levels in the atmosphere prior to the industrial revolution, although I also think that this is such a small factor in the context of burning coal and oil as to be of academic rather than any other interest. It is quite clear that the big take-off in ghg production corresponds to the industrial revolution, whatever small steady climb there was before that.

However I think that prior to agriculture the effects of hunter-gatherers on ghg would have been effectively zero. I pointed out above that changes in climate leading to changes in vegetation lead in turn to natural changes in fire regimes. That is, the base line for assessing hunter-gatherer activities isn’t zero, and that being so I think you would be extremely hard pressed to demonstrate that hunter-gatherer use of fire (even if you accepted that this was significant, which I don’t) had an effect over and above whatever changes in natural fire regimes were occurring.

[Response: Well, I have direct and strong evidence that burning by Native Americans in Yosemite kept forest carbon content below 50% of what it now is, which likely held ~ true through a very large part of mid elevation forests in California, not to mention large areas in other western states. People are missing Geno’s point, which had to do with the forests that never fully develop, from a biomass standpoint, due to frequent fire, whether there was initially a veg type conversion or not. You don’t have to burn forests down to affect terrestrial carbon stores–Jim]

I know Australia isn’t the world, but it is a place often used as the poster child for hunter-gatherers using fire to extensively modify the environment and cause extinctions. The most recent publication, featuring a large number of leading Australian paleobotanists, is as follows:

Extract from the Abstract
“…. On orbital time scales, fire in Australasia predominantly reflects climate, with colder periods characterized by less and warmer intervals by more biomass burning… There is no distinct change in fire regime corresponding to the arrival of humans in Australia at 50 - 10 ka and no correlation between archaeological evidence of increased human activity during the past 40 ka and the history of biomass burning…”

[Response: We’re not talking about orbital time scales David, and one needs to look at the full body of literature to get the picture, not an extract from one abstract. Nor are we discussing megafaunal extinctions–Jim]

“one needs to look at the full body of literature to get the picture” agreed, and there is literature over a number of years (including my own) which supports what the above paper (in itself a review of literature and 28 different charcoal records) is saying. The problem is that we need to get back to a null hypothesis of no effect by hunter-gatherers, and then, if you can demonstrate a particular effect in a particular time and place then good luck to you. The problem for a number of years now is that the reverse process has been followed, in which the null hypothesis is taken to be “of course hunter-gatherers modified the environment by using fire, prove that they didn’t” and I was concerned that the discussion here was following that path.

[Response: Truth by re-framing the null hypothesis? And whatever some may think, there is in fact a much larger literature showing that burning by natives was extensive, which is why it’s the consensus viewpoint. But all of this is frankly, off track. The point relative to this discussion is not whether natives burned or not, but rather how much area burned, regardless of cause.–Jim]

I think there may be huge confusion here about what hunters and gatherers do.

Hunters and gathers are nomadic, They move from place to place frequently in search of games and new plant foods. Quote from Wikipedia:

“Hunter-gatherer societies tend to be relatively mobile, given their reliance upon the ability of a given natural environment to provide sufficient resources in order to sustain their population and the variable availability of these resources owing to local climatic and seasonal conditions. Individual band societies tend to be small in number (10-30 individuals), but these may gather together seasonally to temporarily form a larger group (100 or more) when resources are abundant.”

The California Indians mentioned above were not hunters and gatherers. They were primarily sedentary and resided in villages, often quite large in size, and lived off acorns, salmon, and other abundant resources. Quote from a California government site:

“Like everywhere else, in California, villages were fiercely independent and governed internally, The abundant food supply allowed for the establishment of villages of up to 1000 individuals, including craft speci-alists (spam?) who produced specific objects and goods for a living.”

This is not a description of hunters and gatherers. Of course, like many native peoples around the world, they did hunt and gather but people even in the modern United States hunt and gather today, but that does not make us a hunter gatherer society.

There would be little reason for true hunters and gatherers to clear land because they were not likely to be in the same location again for another year and they would have to clear the land again by that time. On the primarily sedentary population would have great incentive to clear land.

Regarding the extinction of the mega fauna, the reasons for it are a considerable topic of debate but the extinction boundary is more like 10,000 years ago and occurred around the time of the Younger Dryas. There is the comet theory related to this that has since been proven to be unlikely. However, the Younger Dryas, whatever its cause, may be reason enough to explain the extinction, but if we want to introduce the human element, it might have been caused by true hunters and gatherers hunting, not setting the forests on fire.

We may really have more of terminology problem here than a real disagreement. And maybe an under appreciation of the diverse ways in which native peoples lived.

We have true hunters and gatherers that are mobile and in small groups.

When these groups went they came into areas of relatively abundant plant and wildlife, they would be become more sedentary and live in larger groups.

In tropical jungle areas, they might adopt a slash and burn approach moving perhaps once a year or so as the tropical land exposed by fire would lose fertility.

In any case, to support large population densities, they would have to adopt agricultural practices wherever this was possible.

Burning and use of fire would be factor in all of the above except for the true hunter gatherers.

Early inhabitants of the New World probably were hunter gatherers with some settling and becoming sedentary where conditions permitted. Later in the tropical areas there was slash and burn. After that came true agriculture which became prominent in the Mississippian Culture, the Mayan, the Andean cultures, and possibly areas of the Amazon. But this was in the last 2,000 primarily where it became large scale.

Re comments 51 & 52: Thanks for your interest. As for your questions in paragraph 2: The original case for the early anthropogenic hypothesis rested on evidence that CO2 and methane trends in previous interglacials headed downward and were natural in origin, so the upward gas trends late in this interglacial are not likely to be natural. The new evidence for large land use millennia ago points to the same conclusion. As for getting back to ‘normal’: If we stopped emitting methane, the anthropogenic part of the amount now in the atmosphere would be gone in a decade. But CO2 stays a lot longer (see David Archer’s book featured on this site).

Response to post 54: Valid point, although the area from the highlands of Mexico down into the lowlands of Guatemala is often lumped into “MesoAmerica”. Is ‘meso’ different from ‘central’?

55-61: The (interesting) discussion here has veered off farther into burning, and the varying opinions posted again show that scientists disagree. My comment about early fire was not about the megafaunal extinction or (mainly) about hunter-gatherers. Its focus was the time of early farming when CO2 levels were rising fast (6000-3000 years ago). By this time, global population was in the high tens of millions, on its way to reaching 200-300 million by 2000 years ago. So — there were lots of farmers who were not hemmed in too tightly by neighbors and were thus free to burn. [Also — thanks to David Horton for alerting me to a different view about ‘fire sticks’ inAustralia.]

Jim “The point relative to this discussion is not whether natives burned or not, but rather how much area burned, regardless of cause”. I thought the point was the contribution, if any, of human activity to ghg production prior to the industrial revolution. I don’t want to prolong this too much, but the point is important. There tends to be an assumption that in the absence of human activity there is an absence of fire. For the more sophisticated, who realise that there is a natural fire regime (the fires sparked by lightning for example), the assumption is that human burning, even by hunter-gatherers, greatly increased the amount of fire. This is the proposition that I (thanks Bill) and a few others have been arguing against. And Jim I didn’t change the null hypothesis. The original one was that humans had little if any effect on environment in Australia. This null hypothesis was reframed by the early proponents of fire stick farming. From that point on every example of fire activity in the past, every ethnographic record, every environmental change, every apparent “fire adaptation” of plants, was taken as “proof” of Aboriginal use of fire. My view is that while Aborigines obviously used fire, probably caused bush fires on occasion, this fire activity took place within the context of, not as an addition to, the natural fire regime.

Hellen. The difference is that =some= hunter gatherers had a much easier and more pleasant life, obviously not compared to ‘modern man’ in advanced economies, but certainly compared to practically every peasant agricultural existence I know of. Indigenous people living along the mouth of the River Murray spent the warm months along the beaches. Gathering lunch by standing in warm sand wiggling your toes and picking up shellfish in places that are now major holiday destinations sounds pretty good to me. As summer came to a close, they’d move inland up the river, plenty of roots to dig, massive – unbelievably huge – freshwater fish to catch. I doubt these people had much impact on such an abundant landscape, apart from the occasional campfire becoming wildfire.

Whereas the activities of agricultural societies throughout Africa, Europe and the Middle East 5000+ years ago would have been noticeable. Ploughing alone exposes soil carbon to oxidation – and then there is stubble burning – another dislocation of the biological carbon cycle.

A 1.2-meter-long chunk of stalagmite from a cave in northern China recorded the waning of Asian monsoon rains that helped bring down the Tang dynasty in 907 C.E., researchers report on page 940 of this week’s issue of Science. A possible culprit, they conclude: a temporary weakening of the sun, which also seems to have contributed to the collapse of Maya civilization in Mesoamerica and the advance of glaciers in the Alps.

And from Polytechnique Federale de Lausanne
New model of man’s role in climate change
January 24, 2011

The Roman Conquest, the Black Death and the discovery of America — by modifying the nature of the forests — have had a significant impact on the environment. These are the findings of Swiss scientists who have researched our long history of emitting carbon into the environment.http://www.physorg.com/news/2011-01-role-climate.html

When I was researching net primary production (NPP) numbers for plugging into Biochar/Biofuel systems climate potential, looking through Dr. Bill Ruddiman’s work at UVA on legacy CO2 and the agricultural revolution, It’s support of Johannes Lehmann’s previous work at Cornell of a potential 10 GtC, and the added perspective of palioclimatic effects of soil carbon loss, brought together many loose threads for me.

Dr. Dull’s recent work brings even more support, related even closer to practices of Terra Preta soils in the Amazon. The BC, charcoal & pollen evidence is hard to ignore

I’m glad this work by Dr. Dull is getting attention. Together with Dr. William Woods and citing Bill Ruddiman’s work, the pieces of anthropogenic climate change fall into place.

The implications are really important. Dull, et al, argue that the re-growth of Neotropical forests following the Columbian encounter led to terrestrial biospheric carbon sequestration on the order of 2 to 5 GtC, thereby contributing to the well-documented decrease in atmospheric C recorded in Antarctic ice cores from about 1500 through 1750. While the paper does not extend to the medieval maximum, from charcoal in lake bed studies it documents increased biomass burning and deforestation during agricultural and population expansion in the Neotropics from 2500 to 500 years BP, which would correspond with atmospheric carbon loading and global warming 1100 to 650 years BP.

Dr.Dull gives us hard numbers for what Charles Mann has tried to get across to us in “1491”, that we don’t give mankind near enough credit for creating our biosphere. Just as Michael Pollan’s “Botany of Desire” showed us how plants have manipulated us to spread them around the globe, the message of man’s mutuality with nature is more than seeping into the data everywhere.

Since we have filled the air , filling the seas to full, Soil is the Only Beneficial place left.
Carbon to the Soil, the only ubiquitous and economic place to put it.

The discussion has wandered off into issues not raised in the original post, so one more brief response here to recent comments about fire. The clearest evidence that early agricultural humans used fire extensively are their first arrivals on islands: New Zealand (already mentioned), Madagascar, and Pacific islands with drier natural climates and lower elevations (see, for example, Rollet and Diamond, Nature 431: 443-446). I find these cases convincing enough to make me think that the use of fire could have been extensive in many other regions that lack this kind of ‘controlled-experiment’ evidence.

“Most previous modeling simulations relied on the simplifying assumption that per-capita clearance and cultivation remained small and nearly constant during the late Holocene, but historical and archeological data now reveal much larger earlier per-capita land use than used in these models”

Once again, the models got it wrong.

[Response: Once again, the contrarian bypasses his frontal cortex when someone uses a word that triggers a knee-jerk reaction. Clue: the land use changes are an input into the models. – gavin]

[Response: Once again, the contrarian bypasses his frontal cortex when someone uses a word that triggers a knee-jerk reaction. Clue: the land use changes are an input into the models. – gavin]
Well, they admit the models relied on a ‘simplifying assumption’. That makes them wrong in my book (or at least fairly useless).
Oh, by the way Gavin, accusing me of bypassing my frontal cortex is a rather nasty thing to say. Is this the kind of “knee-jerk reaction” you make when you hear a dissenting voice?

[Response: Oh please. If I had a dime for every time someone like you sees the word ‘model’ used and instantly says ‘see the models are wrong’, I would certainly have enough to retire and pay some else to rebut this kind of nonsense. It is not an ‘admission’ that models use simplifying assumptions – it is the whole point. As George Box said “All models are wrong. Some may be useful”, but if you want to play games “Oh my! A scientist said a model was wrong!”, please do it elsewhere. – gavin]

@74
[edit–nope, sorry, you’ve been responded to twice and it’s perfectly clear that you’re here to argue, not to learn. Not going to let this discussion get dragged off topic any further. Accuse me of censorship, or whatever else makes you feel good, if you like–Jim]

Has anyone looked into the different carbon uptake by crops compared to the forest they are replacing? Conversely, what is the increase in CO2 output generated by 7 billion people?

[Response: Yes, lots of work on the former, and for typical agronomic crops on a per unit area basis, there is no comparison. These are largely herbaceous (i.e. non-woody) in nature, so the carbon cycles rapidly rather than accumulating, although how much accumulates can be strongly influenced by the cropping system used, as in e.g. no-till systems. Fruit and nut crops are better. And then there are all the related carbon costs of agriculture (esp. fuel, fertilizer). As for the latter, the effect of human biomass on carbon is a wash–all human respiration being balanced by some (ultimate) photosynthesis somewhere.–Jim]

Jim,
Your first response was fine, but then you seemed to negate it with your second. If human respiration is balanced somewhere, why would combustion not also be balanced somewhere? By that reasoning, all additional atmospheric carbon dioxide would ultimately be balanced.

[Response: No. It is related to the source of the carbon. CO2 from respiration comes from food (photosynthesis) which drew CO2 from the current atmosphere – thus it is carbon neutral. CO2 from the combustion of biomass (as long as the biomass is regrown) is also neutral. However, CO2 from fossil fuel combustion is adding carbon to the atmosphere that was drawn from very ancient sources, and so that is an addition to the active carbon pool. – gavin]

Still much of the evidence you continue to cite of extensive use of fire is coming from the last 2,000 years. New Zealand, for example, was settled only 700 years ago. Even Madagascar settlement dates back only 2,000 years.

Is it mostly speculation that there was extensive use of fire by a large enough number of human populations spread over a large enough area to account the increasing GHG levels 5,000 years ago? Or, do you have evidence that I missed regarding the period prior to the last 2,000 years? Or, do you account for it simply by the spread of agriculture through Europe and Asia?

You’re right about some settlement and migration. otoh, agriculture was well under way in many regions of the Middle East, Egypt, Europe 5000+ years ago. And Australia was settled ~60,000 years ago. Most of those settlements were, unsurprisingly, in the same favourable coastal regions as modern populations – but some stayed in the drier more difficult areas. Fire could be useful for hunting in those regions.

I think we get a bit blinded by our own wildly inappropriate land and energy uses to make good judgments about these historical activities. Firstly, early agriculture (and some current practice) was pretty destructive and inefficient. When we look at how much soil carbon can be exposed and oxidised by ploughing, let alone the dreadful multiple ploughing used in some areas/ times, and add in stubble burning, we can see that these activities can have a large effect. Not the same huge instantaneous effect we can create with our large populations and modern equipment but a steady cumulative effect from persistent, repeated, unrelenting disruption of the biological carbon cycle.

Think of it as the difference between large volcanic eruptions (us) and the constant production of gases from small geo-active regions (them).

DanH is asking about combustion *of biomass*, not fossil fuels as Gavin’s response seems to presume.

If I read Jim’s inline correctly, the answer to Dan’s second comment is that the change from forest to cropping creates a CO2 pulse, since the accumulated biomass C is rapidly dumped into the atmosphere when burned. The annual crops do not accumulate a lot of biomass, so the carbon that they store is much lower. (Jim’s caveats about type of crop apply, of course.)

[Response: Dan’s questions were not perfectly clear;his use of the word “additional”
implies adding new carbon to the pool, i.e. fossil fuel C, and thus Gavin’s (correct) answer. My answer to the 2nd question was just pointing out that human respiration (and the respiration of all organisms on the planet for that matter) adds nothing to the active C cycle–it is all simply the return of atmospheric carbon originally fixed by plants, to the atmosphere. However, I wasn’t quite clear enough there, because when you account for all of the fossil fuels used in modern agricultural production (fuels for plowing, planting, pesticide production and application, harvesting, and product shipping, plus fertilizer production), it’s no longer carbon neutral. Early ag would have also not have been carbon neutral, but primarily for a different reason: carbon release from vegetation and soils due to clearing/burning. It’s also important to mention that various forms of low intensity ag, like organic practices, conservation or no-till, dispersed or “personal scale” ag/hort, etc., all greatly affect the final C ledger. Agriculture has a very big and important effect on current GHG production –Jim]

Presumably that pulse would not be an ‘ongoing’ source of C, but Dr. Ruddiman’s theses is that it’s been taking place over millennia.

You could turn Dan’s argument around, I suppose, and ask the question, well, what about the biomass represented by 7 billion humans? But the math doesn’t work.

As a “back of the back of the envelope” pseudo-calculation, when I was looking at the forest mortality question a few days back, I looked at research on the emerald ash borer, which is killing North American ash trees right now. I found out that the population of ash trees in the US is something like twice the global human population. It’s probably fair to assume that most of those trees mass a good deal more than any human. Of course, that’s just one woodland species, by no means the most numerous, and just one country, which though large still comprises only a few percent of the world’s land area.

Conclusion: forest biomass has to be several orders of magnitude greater than human biomass.

(Just for fun, and as a WAG–“wild-ass guess”–I’d venture 5-6 orders of magnitude (minus 1, plus 3) without looking any of my ‘sub-guesses’ up. But I bet Jim has much tighter estimates not far from his fingertips.)

[Response: I’ve not a clue on the total carbon in humans, but the biomass of all animal life on the planet is not even close to that of plants–Jim]

C in humans shouldn’t be hard: Wikipedia has us at 18% C with a mean body mass of 70 kg, so 7 billion of us would be 12.6 kg x 7 billion, or about 88 billion kg, or 88 million metric tonnes. (Just about what Kuwait emitted in 2007, and they were ranked #47.)

Of course, I made that same tired mistake of comparing C (humans) to CO2 (Kuwait’s 2007 emissions.)

My bad, but it really doesn’t affect the point.

[Response: Right, it doesn’t. And I also should clarify what I said in my comment above about early ag not being carbon neutral: this would be true only when cleared land remains cleared, i.e. in continuous agricultural production, or repeatedly burned, etc.–Jim]

Re comment #72: Forcefully (!) answered by Gavin (#72, 73), who I can only imagine must be up to his keester (probably higher) with ‘gotcha’ comments like this about models; by Kevin (#74); and by Jim (#77), who rightly cut ‘Charles’ off from any more of his irrelevant diversions.

Re comment 79: Again, Gavin answered it.

Re comment 80: In the last 2000 years that encompass these island examples, it is clear that these early farmers used fire extensively immediately upon arriving. But people had been using fire for hundreds of thousands of years in Africa, at least tens of thousands of years in Eurasia, and for more than 10,000 years in the Americas. And in all those places they had been farming for 10,000 years, or close to it. Is it really conceivable that when farmers arrived on these islands they were suddenly seized by a brand-new (previously unprecedented) urge to burn much of their new landscapes?

Re Jim’s reply to comment 84: There is a half-disguised subtlety about the effects of shifting cultivation on carbon emissions. When people clear a new plot of forest in order to farm, there is an obvious and immediate effect on carbon emissions and atmospheric CO2 concentrations because crops and pastures have much less standing carbon stored than forests.
The half-disguised part is the footprint that farmers leave behind in plots previously cleared but now abandoned. From a cup-half-full perspective, the trees growing back on these abandoned plots restore carbon. But the more relevant cup-half-empty perspective notes that those all those previously occupied plots take decades to restore the equivalent carbon of a “mature” forest”. So: it is the cup-half-empty view that matters for carbon emissions and atmospheric CO2. Even though people move around, and forests grow back, they leave a trailing C footprint that is tied to population (non-linearly, as my original post showed).
Some years ago at a meeting, I had this argument with Paul Crutzen, and he disagreed with me. At breakfast the next morning, he came right up to me and said:”You are right!” I suppose this is blatant name-dropping, but it is one of my fondest career memories.

Now that this very interesting thread is getting in to carbon cycle and agriculture more generally, I hope I can ask a (couple of) question(s) not too much off topic:
Is there a good place to read about the effects of modern conventional (“industrialised”) agriculture on carbon in the soils?
Of course the original clearing of forests has released carbon. But how big is the effect of “degrading” the carbon content of the soils today?
I suspect it could be temperature dependent. Has anybody been quantifying what effects global warming will have?

1) I grew up on a farm established ~1850 in Western PA. I have sketches from that time showing house, barn and pasture in front of the barn, from the view of the woods on the property opposite. When the farm was sold for development, the pasture was left clear, but of course the cows had departed a few years earlier anyway. (Hence, that form of tree suppression stopped.)

We visited about 20 years later, and at least to look, one would never know there had been a pasture, because the same dense woods had regrown. [That regrowth speed would not be typical everywhere, of course.]

2) While much of the land had been cleared long ago, there were still a few acres of woods, and sometime trees at the edge needed to be cleared back. How did we do it? Steel axes, chainsaws, and if not cutting for firewood, use tractor to drag wood away to a wood/brushpile to be burned later.

Here’s a Penn State paper “trees of Western Pennsylvania about the history, including possible effects of native Americans and then the realities of tree clearing.
‘To these settler, “the trees were the enemy”‘

To me, in the long run, the final arbiter / accountancy / measure of sustainability will be
soil carbon content. Once this royal road is constructed, traffic cops ( Carbon Board ) in place, the truth of land-management and Biochar systems will be self-evident.

A dream I’ve had for years is to base the coming carbon economy firmly on the foundation of top soils. My read of the agronomic history of civilization shows that the Kayopo Amazon Indians and the Egyptians were the only ones to maintain fertility for the long haul, millennium scales. Egypt has now forsaken their geologic advantage by building the Aswan dam, and are stuck, with the rest of us, in the soil C mining, NPK rat race to the bottom. The meta-analysis of Syn-N and soil Carbon content show our dilemma, from University of Illinois & ISU;https://www.agronomy.org/publications/jeq/articles/38/6/2295

Re comment 87: based on personal observations here in Virginia, regrowth depends on ‘what from’. Clear-cutting of hardwoods leaves stumps and roots that regenerate lots of forest carbon in a couple of decades. These clear-cuts would be like ancient woodlots cut on a rotating basis.
Fully cleared pastures and croplands that are abandoned take longer, and reoccupation probably depends on the kinds of pioneering trees. Here in VA, cedar and locust trees come in, but not right away. I suspect from John’s story that maples and others with their helicopter seeds may be quicker.

Re comment 88: You should look more closely at my original post (paragraphs 7-9). Dorian Fuller and colleagues have a paper about to come online in the Holocene special issue that shows a very large and early expansion of rice irrigation across Asia. Their estimates suggest that CH4 emissions from rice irrigation can explain much (~70%?) of the observed CH4 increase between 5000 and 1000 years ago. They also map the expansion of livestock across Asia and Africa, most of which has occurred since 5000 years ago. Livestock emissions are though to be larger than those from rice irrigation today, and likely were in the past.
Earlier, I had been looking for archeological data on the spread of rice irrigation and ended up publishing with Chinese colleagues a 2008 paper in Quaternary Science Reviews (doi: 10.1016/j.quascirev.2008.03.007). An effort by Li and colleagues had also been underway to determine the spread of rice across southern Asia. Their paper was published in Quaternary International (doi: 10:1016/jquaint.2008.02.009). In my opinion, the recent compilation by Fuller and colleagues, which was very carefully vetted for unambiguous archeobotanical evidence of domesticated irrigated rice, is now the definitive effort.

Wouldn’t this expansion of rice been preceded by burning and CO2 rises?

I don’t really have any concrete evidence of this but it seems that the first step in any new area would be slash and burn which would then be followed by permanent agriculture where crops, climate, and geography make that is possible. The first would lead to CO2 rise and the second more to CH4.

What would have been the effect on atmospheric CO2 of early metalworking?
I suggest that if we could estimate the quantity of ores mined at various times and in various places we could estimate the fuel required to work the metals and the amount of CO2 produced from metals smelted from carbonates.

The production of metals, pottery, glass, lime, leather, soap and many other items requires land and fuel – and all of these processes add to atmospheric levels of CO2. I believe that reasonable estimates could be made for the emissions of CO2 due to early industries and the amount of land used. I suggest that the clearing of trees for better access to the land for surface mining may also be relevant to these studies.

Our farming for over 10,000 years has been responsible for 2/3rds of our excess greenhouse gases. This soil carbon, converted to carbon dioxide, methane and nitrous oxide began a slow stable warming that now accelerates with burning of fossil fuel. The unintended consequence has been the flowering of our civilization. Our science has now realized the consequences and developed a more encompassing wisdom.

This plots that data, basically a subset of the one in #96. Since I’m not good at eyeballing comparing diagonals to see rates of change, I graphed 25,51, and 75-year linear regressions (Excel SLOPEs)to get a simplistic sort of first derivative and ignoring uncertainties in both date and CO2 values. That gives this graph.

One can summarize this as:
1) 1AD-1000AD rate of change only rarely hit +/- 0.05ppm/year over 25,50, or 75-year time-scales.

2) By 1300AD, one starts to see much stronger gyrations at all those scales.

Re comment 92: Yes, CO2 emissions from clearance would have preceded CH4 emissions from rice irrigation, but probably only by years (decades?). Sediment data in the areas of China most favorable for irrigated rice show a decrease in elm pollen, a kind of tree likely to have been growing in wet lowland areas later converted to rice farming.

Re comment 94: As noted earlier, all of those pre-industrial manufacturing activities are thought to have had a much smaller overall effect than clearance for farming, at least until very recent centuries, and then only locally.

Re comment 96: You didn’t take the time to make your point. Anyway: yes, population played a role in the CO2 rise since 1850, but that’s also the time when unprecedented C emissions from fossil fuels add to the previous ones that were mainly from deforestation. The whole point of my original post was to summarize new evidence that those pre-industrial GHG emissions were not linked linearly to population, as has been widely assumed

Those following this discussion might want to check out a recent paper by Mitchell et al in JGR (doi: 10.1029/2010JG001441). They measured decadal-century scale CH4 variations over the last 1000 years in the WAIS (West Antarctic Ice Sheet) ice core, and found variations very close to those measured earlier at the Law Dome site, thus validating both records. They found weak to negligible correlations of CH4 with reconstructed temperature and precipitation, but a suggestive correlation of CH4 minima with anthropogenic episodes: Genghis Kahn’s early-1200’s assault on China and its agricultural infrastructure (less rice irrigation), the mid-1500’s spread of European diseases in the Americas (reduced biomass burning), and the early-1600’s civil unrest and population losses in China. The analysis by Mitchell et al also shows that invoking an anthropogenic explanation for those CH4 dips would require a level of anthro-driven CH4 emissions (the baseline level before those events occurred) that lies at or above the higher end of previous anthropogenic estimates (mine included).